1. Field of the Invention
The present invention relates to a communication device, in particular to a spread-spectrum communication device capable of eliminating a narrow-band interfering signal.
2. Description of the Related Art
Spread-spectrum techniques expand bandwidth to gain transmission advantages. At a transmitter, pseudorandom spreading distributes the transmitter's power over a much wider frequency range, with much more power density. Because the spreading is reversed at a receiver, narrow-band interfering signals are spread before demodulation, and wide-band interfering signals remain wide-band. The interference power density in the reconstructed narrow band remains low, while the higher power density of the desired signal is available to the receiver demodulator. Therefore, interference is reduced.
FIG. 7 shows one example of a conventional spread-spectrum communication device. The spread-spectrum communication device 100 includes an antenna 101 for receiving a spread signal, a bandpass filter 102, an amplifier 103, a circuit 104 for eliminating a narrow-band interfering signal, a local signal source 105, a mixer 106, a bandpass filter 107, an amplifier 108, a matched filter 109 for reversing the spread signal, a demodulator 110, and an output terminal 111.
FIG. 8 shows one example of the eliminating circuit 104 which includes a switch 121, band elimination filters BEF1-BEFn, bandpass filters BPF1-BPFn, detectors DET1-DETn, a circuit 125 for determining a maximum-value of the outputs of the detectors DTE1-DTEn, a switching circuit 126, an input terminal 127, and an output terminal 128. The inputs of bandpass filters BPF1, BPF2, . . ., BPFn, are together connected to the input terminal 127, and their outputs are connected to the determining circuit 125 via the respective detectors DET1, DET2, . . ., DETn. The switch 121 has its input connected to the input terminal 127 and a plurality of outputs connected to the filters BEF1, BEF2, . . ., BEFn. The outputs of the band elimination filters are connected together to the output terminal 128. The groups of band elimination filters 122, the bandpass filters 123, and the detectors 124 respectively include the same number of filters.
In the spread-spectrum communication device 100, a signal received by the antenna 101 is fed to the bandpass filter 102 in which unnecessary frequency components of the signal are removed. The signal is then amplified by the amplifier 103, and fed to the circuit 104.
In the circuit 104, respective band pass filters have different frequency bands, and the bands of adjacent filters are consecutive. As a result, band pass filters BPF1-BPFn, as a whole, pass any signal as long as the frequency of the signal is within the spread frequency band. Similarly, the respective band elimination filters eliminate signals in respective frequency bands.
Further, in the respective couples of band pass and band elimination filters, e.g. BPFl-BEF1, BPF2-BEF2 . . ., both of the filters have same band.
An input spread signal from the terminal 121 proceeds to the band pass filters. Then, the detectors detect levels of the spread signal at their own band frequency. The circuit 125 determines a signal having a maximum level among the output signals from the detectors. In response to the determination, the switch 121 controls the switch to feed the input signal into a band elimination filter which corresponds to the band pass filter whose output has the maximum signal level.
Assuming that the level of an interference signal is extremely larger than the level of the spread signal, the interference signal can be eliminated by the above-process.
The signal is input to the frequency converter 106 via the output terminal 128, and further input to the bandpass filter 107 to remove unnecessary frequency components. The signal is then amplified by the amplifier 108, and despread by the matched filter 109, demodulated by the demodulator 110, and then output to the output terminal 111.
The narrow-band interfering signal eliminating circuit 104 involves a number of bandpass filters and band elimination filters, which are costly and increases space requirements. When the number of narrow-band interfering signals is more than one with their frequencies dispersed among different bands, only the highest level interfering signal is eliminated, with the remaining ones left unprocessed. The S/N ratio of the despread signal is thus degraded.